Scared of a phantom leaving for a ball (6)

I believe the answer is:

aghast

I'm a little stuck... Click here to teach me more about this clue!

'scared' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for aghast that I've seen before include "Appalled, shocked" , "Filled with fear or horror" , "What's carried in bag has teacher" , "Stupefied with horror" , "Filled with consternation or horror" .)

I've seen this clue in The Hindu.

Want a hint initially instead of a full solution? Install my app